In a typical plural color thermal printer, successive sheets of paper are fed from a sheet source to a platen such as a rotary drum. Each sheet is wrapped around the drum and rotated into position opposite a thermal print head located adjacent to the drum. The print head has a plurality of addressable vertical wires and a single horizontal wire on its surface closest to the drum. By sending current to the horizontal wire and one of the addressable wires, the intersection of the two wires can be heated at a selected point along the length of the drum.
A spooled print ribbon having a plurality of color bands in a repeatable sequence is disposed to pass between the print head and the drum. Often, the ribbon has color bands corresponding to the primary subtractive colors cyan, magenta and yellow and sometimes black. The colors are present on the ribbon as a thin heat-transferable coating on the side of the ribbon facing the drum.
In a typical thermal printer, after a sheet of paper is wrapped around the drum, the drum is rotated to position the sheet at a "top-of-sheet" position opposite the print head. Also, the print ribbon is positioned so that the top of the first color band is located at the top of the paper sheet. A mechanism then moves the print head so that it presses the ribbon and paper sheet against the drum. Immediately thereafter, the print wires of the print head are addressed sequentially across the head according to control signals from a controller representing a line of print information. The wires of the print head are thus heated at selected pixel locations or points along the drum causing spots or dots of wax of the first color on the print ribbon to be melted into the paper sheet along the first line to be printed. Next, the paper sheet and ribbon are advanced one line by rotating the drum and ribbon spool. The wires of the print head are again energized selectively by the controller to print the second line of dots of the first color on the paper sheet. This print-and-feed sequence is repeated until the sheet is completely printed with the first color.
Following this, the head is retracted from the ribbon and the drum is rotated so that the top of the sheet is again opposite the print head and the print ribbon is advanced to place the top of its second color field in alignment for printing on the first line of the sheet. The head is then repositioned against the ribbon and the above-described print-and-feed sequence is repeated until the paper sheet has been completely printed with the second color, after which the sheet is again returned to the top-of-sheet position opposite the print head to receive the third color. This process is continued until all of the colors have been printed on the sheet. The printed sheet is then removed from the drum to be replaced by the next sheet to be printed on.
It can be appreciated from the foregoing discussion that, when printing on each sheet, the print head must be removed repeatedly between its "print" position wherein it presses the print ribbon and paper sheet against the drum and its "feed" position wherein the print head is retracted from the drum so that the print ribbon and drum can be moved relative to the print head. Such movements occur when the sheet is mounted to the platen and positioned for printing the first color, when the sheet is repositioned prior to printing each successive color and after completion of printing when the sheet is being removed from the drum.
Invariably, in printers of this general type, the print head is shifted between its aforesaid two operative positions by a solenoid and return spring type of mechanism. The print head is moved to its print position against the drum when the printer controller energizes the solenoid and the print head returns to its retracted feed position when the current applied to the solenoid is reduced to the point where the force of the solenoid is overcome by the return force of the spring. The use of this type of mechanism to move the print head has several disadvantages. More particularly, in order to move the print head promptly and reliably, the solenoid has to be quite massive and heavy so that its presence has a pronounced effect on the size and weight of the printer as a whole. Also, the solenoid has to be on or energized during the entire print cycle and this increases considerably the overall power requirements of the printer. Such a large solenoid also creates excessive heat in the printer which can degrade the performance of the print ribbon and other components of the printer. Additionally, in some prior printers, the movements of the head are not controlled closely enough. Resultantly, the head may sometimes be instructed to print when it is not quite in its print position or the drum may be advanced when it is still engaged by the head, thus spoiling the quality of the printing or causing a jam.